Low energy consumption and long battery life are important qualities to consider in designing and operating wireless networks. One way of reducing the networked devices' energy consumption is to enable the devices to go to inactive or sleep mode when possible, but without disrupting the network operation. It would be beneficial to lower the duty cycle of nodes while still achieving the goal of the network operation.
Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards specify physical layer and medium access control (MAC) layer protocols for wireless local area networks (WLANs). The basic service set (BSS), which is a group of devices that communicate with one another, is the building block of an IEEE 802.11 network. The BSS can be realized in two ways—independent BSS and infrastructure BSS. An infrastructure BSS has an access point, and the BSS is naturally delineated and limited by the radio reachability to and from the access point. Prior to the 802.11s standard, IEEE 802.11 standards did not provide packet routing for ad hoc networks, and it is difficult to coordinate power saving (sleep) modes of the devices except for the case of infrastructure BSS. It has been reported that 802.11 is not appropriate for applications such as the sensor networks, which need low duty cycle. In 802.11, nodes' idle listening consumes much energy.
Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standards specify the physical layer and medium access control (MAC) layer protocols for low-rate wireless personal area networks (LR-WPANs). The IEEE 802.15.4 standards are also commercially popular for wireless sensor networks. The IEEE 802.15.4 MAC provides two modes of operation—the beacon-enabled (synchronous) mode and the beaconless (asynchronous) mode. In the beacon-enabled mode, synchronization through transmission and reception of beacons allows devices to get into energy-saving (sleep) mode between coordinated transmissions, and thus helps reduce energy consumption. In a multi-hop network, the beaconless mode requires nodes to always listen for other nodes' transmission. IEEE 802.15.4 classifies devices into Full Function Devices (FFDs), which can broadcast their beacons, and Reduced Function Devices (RFDs), which cannot broadcast beacons. In the beacon-enabled mode, devices can coordinate data transmission with one another.
A main advantage of IEEE 802.15.4 standards is its low energy consumption. However, 802.15.4 networks can suffer from a serious reliability problem if a contention-based medium access control (MAC) protocol is used. It has been reported that a very low packet delivery ratio is observed whenever the power management mechanism is enabled. For reliability, Guaranteed Time Slots (GTSs) can be used in the beacon-enabled (synchronous) mode. However, IEEE 802.15.4 in beacon-enabled mode does not define when full-function devices (FFDs) are to broadcast their own beacons in multi-hop topologies, and thus the standards do not present how the nodes form a synchronous cluster-tree network. In addition, the medium access control of IEEE 802.15.4 does not solve the hidden node problem, and the hidden node problem can seriously degrade network performance and waste energy when the traffic load is heavy.
Bluetooth Low Energy (BLE) is a feature of Bluetooth version 4.0 and is aimed at principally low-power applications of wireless devices. In some cases, Bluetooth Low Energy enables products to operate for more than one year without recharging the battery. BLE is optimized for non-continuous traffic and is suitable for devices that periodically transmit data, such as sensors that periodically report their sensor data. However, the Bluetooth Low Energy standard does not support “scatter net” formation. BLE only allows for “piconet” formation—all nodes in the network should be within the radio range of the cluster head (master) node. The radio range of BLE devices is 50 meters. If devices are distributed over distances much more than 100 meters in diameter, a BLE network is not suitable.
As energy efficiency is an important design criterion for wireless sensor networks (WSN), a number of wireless sensor network protocols have been expressly designed for that purpose. In many WSN applications, devices are idle for a large portion of time, but nodes that are idle yet listening still consume much energy. As idle-listening is a major reason for energy waste, the S-MAC and T-MAC protocols, which are respectively found in
Wei Ye, John Heidemann, and Deborah Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor Networks,” in Proceedings of the 21st International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, N.Y., USA, June, 2002; and
T. Dam, K. Langendoen, “An Adaptive Energy-Efficient MAC Protocol for Wireless S-ensor Networks”, in Proceedings of the first ACM Conference on Embedded Networked Sensor Systems (SenSys), 2003,
are designed to make the duty cycles of the nodes low. In S-MAC and T-MAC, each node repeats a cycle of active state and sleep state, and nodes attempt to synchronize their active states by exchanging SYNC packets. During their active state, medium access control is contention-based. In order to avoid packet collisions, nodes use Request-to-Send (RTS) and Clear-to-Send (CTS) packets. Z-MAC, which is presented in                I. Rhee, A. Warrier, M. Aia, and J. Min “ZMAC: A Hybrid MAC for Wireless Sensor Networks”, in Proceedings of the 3rd ACM Conference on Embedded Networked Sensor Systems, (SenSys), 2005,is a hybrid MAC protocol that combines TDMA and CSMA. Z-MAC performs time slot assignment at the time of deployment, so high overhead is incurred at the beginning. In each assigned time slot, nodes are required to perform carrier sensing in order to avoid collisions. The node to which the time slot is assigned (the owner of the time slot) exercises higher priority in carrier sensing by adjusting the initial contention window size in such a way that the owner of the time slot has an opportunity to transmit earlier than non-owners.        
In order to avoid application packet collisions, the Powermesh MAC protocol, which is presented in
E. Seyedin., D. C. Lee, S. Yang, E. Lee, D. Boone, M. Steiner-Jovic, “Powermesh medium access control protocol”, in Proceedings of the 5th International Conference on Signal Processing and Communication Systems (ICSPCS), 2011,
separates signals from different devices in the time domain through scheduling. Powermesh allows devices to dynamically schedule their transmission and reception times in a distributed manner. Scheduling is performed based on the devices' local knowledge of the current schedule in the vicinity and the devices' need for transmitting application packets. Devices can be in a sleep mode when not scheduled for transmission or reception. In the Powermesh protocol, devices cannot distinguish whether a time slot is scheduled for transmission and reception of a control packet or a data (application) packet, and this ambiguity makes it difficult to detect and repair conflicting schedules.